1. Field of the Invention
The present invention relates generally to the use of supercritical fluids in cleaning or extraction processes. More particularly, the present invention relates to a continuous or semi-continuous process wherein the main cleaning or extraction chamber used in the process is continuously pressurized with supercritical fluid during the cleaning or extraction process.
2. Description of Related Art
Conventional solvent-aided cleaning and extraction processes are currently being re-evaluated due to problems with air pollution and ozone depletion. In addition, recent environmental legislation mandates that many of the organic solvents used in these processes be banned or their use severely limited. The use of supercritical fluids for cleaning or extracting a wide variety of materials has been under investigation as an alternative to the above-mentioned solvent based processes. A supercritical fluid is a dense phase gas which has been compressed to either supercritical or subcritical conditions to achieve liquid-like densities. Unlike organic solvents, such as hexane or 1,1,1-trichloroethane, supercritical fluids exhibit unique physical properties such as low surface tension, low viscosity, high diffusivity, and variable solute carrying capacity.
The solvent properties of compressed gases are well known, as discussed in U.S. Pat. No. 5,068,040, assigned to the present assignee. In the late 1800's, Hannay and Hogarth found that inorganic salts could be dissolved in supercritical ethanol and ether (J. B. Hannay and H. Hogarth, J. Proc. Roy. Soc. (London), 29, p. 324, 1897). By the early 1900's, Buchner discovered that the solubility of organics such as naphthalene and phenols in supercritical carbon dioxide increased with pressure (E. A. Buchner, Z. Physik Chem., 54, p. 665, 1906). Within forty years Francis had established a large solubility database for liquified carbon dioxide which showed that many organic compounds were completely miscible. (A. W. Francis, J. Phys. Chem., 58, p. 1099, 1954).
In the 1960's there was much research and use of dense phase gases in the area of chromatography. Supercritical fluids (SCF) were used as the mobile phase in separating non volatile chemicals (S. R. Springston and M. Novotny, "Kinetic Optimization of Capillary Supercritical Chromatography using Carbon Dioxide as the Mobile Phase", CHROMATOGRAPHIA, Vol. 14, No. 12, p. 679, December 1981). Today the environmental risks and costs associated with conventional solvent aided separation processes require industry to develop safer and more cost-effective alternatives. The volume of current literature on solvent-aided separation processes using dense carbon dioxide as a solvent is evidence of the extent of industrial research and development in the field. Documented industrial applications utilizing dense fluids include extraction of oil from soybeans (J. P. Friedrich and G. R. List and A. J. Heakin, "Petroleum Free Extracts of Oil from Soybeans", JAOCS, Vol. 59, No. 7, July 1982), decaffination of coffee (C. Grimmett, Chem. Ind., Vol. 6, p. 228, 1981), extraction of pyridines from coal (T. G. Squires, et al., "Super-critical Solvents. Carbon Dioxide Extraction of Retained Pyridine from Pyridine Extracts of Coal", FUEL, Vol. 61, November 1982), extraction of flavorants from hops (R. Vollbrecht, "Extraction of Hops with Supercritical Carbon Dioxide", Chemistry and Industry, 19 Jun. 1982), and regenerating absorbents (activated carbon) (M. Modell, "Process for Regenerating Absorbents with Supercritical Fluids", U.S. Pat. No. 4,124,528, Nov. 7, 1978).
Examples of processes utilizing supercritical fluids are described in U.S. Pat. Nos. 5,013,366 and 5,068,044 which issued on May 7, 1991 and Nov. 26, 1991 respectively, and are assigned to the present assignee. The contents of these two patents is hereby incorporated by reference. The processes described in the two patents are typical of accepted batch type processing wherein the substrate, sample or other material to be processed is loaded into a processing vessel which is unpressurized. The vessel is subsequently sealed and then pressurized with supercritical fluid. After completion of the cleaning or extraction process, the vessel is depressurized and opened for sample unloading. While useful for precision cleaning of various substrates, this type of batch processing is labor intensive and requires long pressurization and depressurization times. In addition, the pressurization vessels used in batch-type processes must be relatively large in order to achieve even moderate volumetric throughput. These large pressurization vessels are expensive.
It would be desirable to provide a supercritical fluid processing system which is more efficient, quicker and less labor intensive than the above-described batch-type processes. Further, it would be desirable to decrease the size of the pressurization vessel in order to decrease the cost and improve the efficiency of supercritical fluid processing where large volumetric throughput is required.